This invention relates to a method and a device for forming a pipe in which a plate with holes is formed into a completely circular pipe by use of a bending machine.
A method of forming a completely circular pipe by bending a steel plate by use of a bending machine has heretofore been performed with a bending machine in which one upper roll is vertically and horizontally movably arranged over two lower rolls arranged parallel to one another. A pipe forming method disclosed in JP patent publication 63-36852 is one example. This prior method comprises a preparatory step of bending a central portion, a main step and a step of bending both ends of a plate.
In the preparatory step of bending the central portion of the plate, the upper roll is lowered offset relative to the two lower rolls to a position where frictional force necessary for feed of the plate is obtained. Thereafter, the upper roll is further lowered while pressing and bending the plate for rolling the plate until a predetermined arc necessary for the main step is obtained. After performing the main step in which rolling is further performed with the predetermined arc, the both end bending step is performed in which pressing/bending is applied to both ends of the plate.
With this bending method, it is possible to bend a steel plate into a U-shape using part of its working steps. One example of such a method is proposed in JP patent publication 2000-288635. In this U-bending method, after moving a steel plate clamped between an upper roll and two lower rolls to a predetermined forming start position, the upper roll is lowered with a lowering amount divided into a plurality of portions so as to form a predetermined arc, while rotating the lower rolls in normal and reverse directions at rotating amounts corresponding to lowering amounts to perform multiple step formation while gradually narrowing each formation toward a minimum curvature portion.
Bending methods by use of the bending machines described in these two patent publications are used for flat plates in which no holes are formed. No mention is made about what influence holes being formed in a flat plate will have on work accuracy when forming the flat plate into a pipe. If a pipe is actually formed by applying the bending method of JP patent publication 63-36852 to a work with holes, it is known from experience that no normal bending is possible near portions where the holes are present, and a radius of curvature partially decreases at such portions and the pipe is not made to be completely circular.
Thus, in forming a pipe with holes, in order to obtain a completely circular pipe, a plate has to be treated by one of the following two methods. That is, the pipe forming method of JP patent publication 63-36852 is applied without forming any hole in the plate to form a completely circular pipe, and thereafter, holes are formed at predetermined positions by use of a drill. Otherwise, after a pipe has been formed by applying this pipe forming method to one in which holes have been formed in a work beforehand, bending is performed again with a different kind of press device to correct curvature of the pipe, which is out of true at hole portions.
But, simply by applying the pipe forming method of JP patent publication 63-36852, it is impossible to obtain a completely circular pipe. If holes are formed with a drill after forming a completely circular pipe, the pipe may become partially not completely circular due to influence of drilling. If a material formed with holes is formed into a pipe, it may develop portions which are partially not completely circular. Thus, it is necessary to perform bending again at such portions. Thus, extremely complicated steps are needed. So it is difficult to obtain a completely circular pipe with a bending machine alone. A different kind of machine and step are needed for correction.
An object of this invention is to provide a method and a device for accurately and efficiently forming a pipe with a bending machine only, without using correctional measures, by uniformly bending a plate with holes to a pipe diameter.